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Hawaiian Islands Synoptic Discussion and Guidance

018
FXHW60 PHFO 280135
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 PM HST SAT AUG 27 2016

.SYNOPSIS...
Locally breezy trade winds will continue through early next week as
high pressure persists north of the state. Passing showers will
favor windward and mauka areas, with showers more active tonight
through Sunday night. By the middle of next week, we may begin to
feel impacts from Tropical Storm Madeline.

&&

.DISCUSSION...
Water vapor imagery shows an upper level ridge north of the
state, with a low far to the northwest. At the surface, high
pressure is located far to the north, with a ridge extending to
the south- southwest. 00z soundings from Lihue and Hilo show
inversions just under 9kft and precipitable waters of 1.25 and
1.34 inches respectively. Early afternoon MIMIC total
precipitable water imagery shows an area of moisture near Maui
County/Big Island extending 200 miles to the northeast, with drier
air across Kauai/Oahu. A wedge of drier air is also located
between 50 and 150 miles east of the Big Island, with one surge of
tropical moisture centered near 150W and a larger one near 140W
associated with Madeline.

Trade winds will be the dominant feature for the next few days.
Even though the high is far north of the state, a nearly
stationary low to our south will help to enhance the pressure
gradient across the islands. This will lead to locally breezy
conditions, especially from tonight into Monday. Rainfall has been
greatest across Oahu/Maui County with the wettest gages receiving
a quarter to a half inch of rain over the past 12 hours. The
least rainfall occurred over Kauai, where gages only measured a
couple hundredths of an inch.

An area of showery clouds is located within the pocket of higher
moisture northeast of the state. Showers have already started to
increase across Maui County, and will spread across the rest of
the state tonight. High-resolution model guidance shows a surge in
trade winds with this feature, and forecast soundings/cross
sections show an increase in moisture depth. Rainfall chances will
be highest across windward sections, but given the higher
inversions and locally breezy trades we will see an increase in
showers across leeward sections as well. Behind this feature,
moisture depth starts to decrease Sunday night, and we may be
drier than normal by Monday. Another surge of moisture to enhance
trade showers is possible on Tuesday, but that is starting to get
into the more uncertain period of the forecast.

The National Hurricane Center in Miami is currently issuing
bulletins on Tropical Storm Madeline, which is located just east
of 140W. Madeline is expected to move northwest for the next few
days before turning west, and may impact the state during the
middle of the week. There has been a lack of consistency in model
guidance with this system, leading to greater than normal
uncertainty in the track and intensity forecast. This may result
in significant changes to the types of impacts we could see
across the state. Be sure to stay up to date with the latest
forecasts, which are issued every six hours. When Madeline crosses
140W into the Central Pacific Basin this evening, the Central
Pacific Hurricane Center in Honolulu will begin issuing
advisories. In addition, Hurricane Lester is located far to the
east, but may affect the state over the upcoming holiday weekend.
Given it's more extreme distance, there is even greater
uncertainty as to possible impacts to the state from Lester.

&&

.AVIATION...
Tradewind moisture will affect mainly windward and mountain areas
of the islands occasionally this weekend. Some MVFR ceilings and
visibilities are expected with the clouds and showers passing
through. But overall...VFR conditions will prevail around the
state. Cloud tops are running mostly between 9000 and 11000 feet.
Currently there are no AIRMETS or SIGMETS.

&&

.MARINE...
A Small Craft Advisory remains in effect for the typically windier
locations around Maui county and the Big Island, due to locally
breezy trade winds accelerating around the island terrain. The
morning ASCAT pass missed the windier areas, but otherwise showed
winds on the lighter side today. Winds are expected to pick up
tonight as the cloud feature northeast of the state moves through.

No significant swells are expected through the middle of next week,
though surf along east facing shores will be slightly elevated
due to the stronger trades. Winds and seas, as well as surf along
east facing shores, may increase toward the middle and end of next
week due to the possible effects from Tropical Cyclones Madeline
and Lester. Uncertainty in this forecast scenario is high.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

Discussion/Marine...Bravender
Aviation...Fujii

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office